The following describes a related-art technology associated with the present invention with reference to FIGS. 4 through 7. FIG. 4 shows the record driving circuit of a magnetic head. FIG. 5 shows the waveforms of the voltages and data at both output ends when the magnetic head is normally operating. FIG. 6 shows a related-art magnetic-head short-circuit detector. FIG. 7 is a diagram for describing the detection of head short-circuit.
First, driving of the magnetic head of a HDD (Hard Disk Drive) for example will be described. As shown in FIG. 4, a transistor Q.sub.1 is connected at the emitter thereof to a transistor Q.sub.3 at the collector thereof and a transistor Q.sub.2 is connected at the emitter thereof to a transistor Q.sub.4 at the collector thereof. The collectors of the transistors Q.sub.1 and Q.sub.2 are connected to a power supply V.sub.CC and the emitters of the transistors Q.sub.3 and Q.sub.4 are grounded through a current source I.sub.w. In such a circuit constitution, both ends H.sub.X and H.sub.Y of the coil of the magnetic head are connected to the connection point between the transistors Q.sub.1 and Q.sub.3 and to the connection point between the transistors Q.sub.2 and Q.sub.4 respectively. It should be noted that L.sub.H denotes magnetic-head inductance and R.sub.L denotes magnetic-head resistance.
In the above-mentioned circuit constitution, the transistors Q.sub.1 through Q.sub.4 are controlled in switching based on write data to apply an electric current to the magnetic-head coil, thereby generating a recording field to record data onto a magnetic recording medium. Namely, turning on the transistors Q.sub.1 and Q.sub.4 when the data with go to a logic high level (hereafter referred to as HIGH) and turning off the transistors Q.sub.2 and Q.sub.3 when the data go to a logic low level (hereafter referred to as LOW) cause current i.sub.W1 to flow. On the other hand, turning on the transistors Q.sub.2 and Q.sub.3 when the data goes HIGH and turning off the transistors Q.sub.1 and Q.sub.4 when the data goes LOW cause current i.sub.W2 to flow, generating an inverted field for magnetic recording. It should be noted here that i.sub.W1 and i.sub.W2 are derived from the current source I.sub.W and take a same value and V.sub.2 represents a potential difference between the magnetic head terminals H.sub.X and H.sub.Y and has a relation of V.sub.2 =i.sub.W1 R.sub.L =i.sub.W2 R.sub.L.
FIG. 5 shows the waveforms of write data (WD), voltages V(H.sub.X) and V(H.sub.Y) at the terminals H.sub.X and H.sub.Y, and charge voltages V(TP.sub.11) and V(TP.sub.12) at test points TP.sub.11 and TP.sub.12 in the conventional magnetic-head short-circuit detector to be described with reference to FIG. 6.
When the write data is HIGH, the voltage V(H.sub.X) at the terminal H.sub.X becomes V.sub.CC -V.sub.BE, namely a voltage obtained by subtracting a voltage drop due to the transistor Q.sub.1 shown in FIG. 4 from the supply voltage V.sub.CC and the voltage V(H.sub.Y) at the terminal H.sub.Y becomes V.sub.CC -V.sub.BE -V.sub.2, namely a voltage obtained by further subtracting a voltage drop V.sub.2 due to the current i.sub.W1 from the above-obtained voltage V.sub.CC -V.sub.BE. Likewise, when the write data is LOW, the voltage V(H.sub.Y) at the terminal H.sub.Y becomes V.sub.CC -V.sub.BE, namely a voltage obtained by subtracting a voltage drop due to the transistor Q.sub.2 shown in FIG. 4 from the supply voltage V.sub.CC and the voltage V(H.sub.X) at the terminal H.sub.X becomes V.sub.CC -V.sub.BE -V.sub.2, namely a voltage obtained by further subtracting a voltage drop V.sub.2 due to the current i.sub.W2 from the above-obtained voltage V.sub.CC -V.sub.BE. It should be noted that a flyback pulse is caused due to a current component that rushes into the magnetic-head inductance LH at switching of each transistor.
In such a recording operation as described above, various monitoring capabilities for monitoring the states of the operation are required of the driving circuit, one of the capabilities being the detection of a short-circuit on a magnetic head. This capability outputs a signal indicative of an abnormal operation if the magnetic-head coil shorts for some reason.
FIG. 6 shows an example of a conventional magnetic-head short-circuit detector, also providing a frequency detecting capability. A terminal H.sub.X of the magnetic head is connected to the base of a pnp-transistor P.sub.1. A current source I.sub.1 is inserted between a power supply V.sub.CC and the emitter of the transistor P.sub.1. This constitution charges a capacitor C.sub.1. Likewise, a terminal H.sub.Y is connected to the base of a pnp-transistor P.sub.2. A current source I.sub.2 is inserted between the power supply V.sub.CC and the emitter of the transistor P.sub.2. This constitution charges a capacitor C.sub.2.
As shown in FIG. 5, the flyback pulses caused at the terminals H.sub.X and H.sub.Y at switching between HIGH and LOW of the write data operate the transistors P.sub.1 and P.sub.2 to discharge the capacitors C.sub.1 and C.sub.2, which are then charged again by current i.sub.1 and i.sub.2. Therefore, the charge voltage waveform of the capacitor C.sub.1, namely the voltage waveform of the test point TP.sub.11 is represented by V(TP.sub.11) shown in FIG. 5 and the charge voltage waveform of the capacitor C.sub.2, namely the voltage waveform at the test point TP.sub.12 is represented by V(TP.sub.12) shown in FIG. 5.
The voltage at this test point TP.sub.11 is inputted in a comparator 1 and the voltage at the test point TP.sub.12 is inputted in a comparator 2 to be compared with a common voltage V.sub.TH =V.sub.CC -V.sub.1 that provides comparison reference. If, as shown in FIG. 7, flyback pulses are caused at period T for discharging, charging starts after a flyback pulse to supply charge from the current source I.sub.1 to the capacitor C.sub.1 and from the current source I.sub.2 to the capacitor C.sub.2, raising the voltage with time. In this state, when discharge is caused by a next flyback pulse before the charge voltage reaches V.sub.TH, only LOWs are kept outputted from the comparators 1 and 2. Therefore, LOW is kept outputted from an OR gate 5, indicating a state in which no short-circuit is taking place on the magnetic head.
On the other hand, if a short-circuit is taking place on the magnetic head, the terminals H.sub.X and H.sub.Y both present the same potential as V.sub.CC -V.sub.BE and are fixed thereto. In this state, the transistors P.sub.1 and P.sub.2 do not operate, while the capacitors C.sub.1 and C.sub.2 are kept charged to raise the voltages at the TP.sub.11 and the TP.sub.12, being held over the voltage V.sub.TH. Consequently, HIGH is kept outputted from the comparators 1 and 2 and therefore HIGH is kept outputted from the OR gate 5, indicating a state in which a short-circuit is taking place on the magnetic head. It should be noted that a flip-flop 4 synchronizes with the write data WD to properly detect the output of the OR gate 5.
The magnetic-head short-circuit detector shown in FIG. 6 also provides a frequency detecting capability. To be more specific, if the period T of the write data WD slows to T+delta T as shown in FIG. 7, it takes longer for the capacitors C.sub.1 and C.sub.2 to be charged. Therefore, the capacitors C.sub.1 and C.sub.2 are charged beyond the comparison reference voltage V.sub.TH. As a result, HIGHs are outputted from the comparators 1 and 2 for a time from t.sub.1 at which the V.sub.TH has been exceeded to t.sub.2 at which discharging is performed by flyback pulse, thereby indicating that the period of the write data WD is longer, or the frequency is lower.
Actually, however, mounting a magnetic head onto equipment requires signal input/output wiring, which inevitably causes an inductance component, thereby generating short flyback pulses. If these flyback pulses take place on the LOW side, they operate the transistors to periodically discharge the capacitors, possibly failing the detection of a head short-circuit.
It is therefore an object of the present invention to correctly detect an electric short-circuit between the terminals of a magnetic head by eliminating the influence of the inductance caused by circuit board wiring.